An inkjet recording apparatus of conventional art is mounted in a printer, a facsimile machine, and a copier, and used as recording means which records an image onto a recording medium in the form of a paper sheet or a thin plastic sheet on the basis of image information. Such an inkjet recording apparatus conducts recording with a print head ejecting ink droplets and therefore has features that the recording means can be easily downsized, that dense images can be recorded at high speed, that the running cost is low, and that the level of noise generated thereby is low owing to the non-impact system. Moreover, the inkjet apparatus has also an advantage that color images are easily recorded with use of multicolor ink.
Drive sources of the inkjet recording apparatus include, for example, a carriage motor which reciprocates and drives a carriage having a print head, an automatic sheet feeder (ASF) motor which feeds a recording medium to a print position, a recovery motor which performs head-cleaning, and a conveyance motor which feeds a recording medium for each print scanning. In the conventional art, a stepping motor is frequently used as the above drive source for such reasons as that it is easy to reduce the cost and that the control is simple.
As mentioned above, the inkjet recording apparatus has a low noise level since it adopts the non-impact system. However, a direct-current (DC) motor is now more frequently used as the above drive source for the purpose of further noise reduction or the like. In order to control this DC motor, a servo control is applied.
FIG. 34 is a block diagram showing electrical configuration of a servo control system 1 for use in the inkjet apparatus of the conventional art. Command voltage acting as a control command is inputted by a servo controller 3 to a DC motor 2 which is to be controlled, and on the basis of the command voltage, a motor shaft rotates. The motor shaft has its rotation angle and rotation speed respectively measured by a speed meter 4 and a position meter 5. The measured position information and speed information are fed back to the servo controller 3, and on the basis of provided target position and target speed, the servo controller 3 outputs command voltage for controlling the DC motor 2. The speed meter 4 and the position meter 5 are realized by a later-described encoder 6.
FIG. 35 is a view schematically showing a configuration of the encoder 6 of the conventional art.
In the encoder 6, a detector 8 detects light emitted by a light emitting diode (abbreviated as LED) 7 through a code wheel 9 so that signals are generated. The code wheel 9 is composed of light-transmitting parts 9a and non-light-transmitting parts 9b each of which has a predetermined distance L. In the detector 8, photodiodes 8a are arranged at predetermined distances, and light detected by each photodiode 8a is converted into an electric signal which is then outputted, with the result that the outputted electric signal is outputted by a comparator 10 as differential output signals 11.
FIG. 36 is a view showing a waveform of the differential output signal 11 outputted by the comparator 10 and waveforms of two electric signals 12a and 12b inputted to the comparator 10. The differential output signal 11 has the waveform which is turned over at intersections 13 of the two electric signals 12a and 12b outputted by the respective photodiodes 8a. Now, in the case where the speed is constant, a duty cycle of the differential output signal 11 will be theoretically 0.5. However, the duty cycle changes with various factors. One of the major factors is a difference in sensitivity of the photodiodes 8a. 
FIG. 37 is a view showing a waveform of the differential output signal 11 outputted by the comparator 10 and waveforms of the two electric signals 12a and 12b inputted to the comparator, which shown waveforms are produced in the case where there is a difference in sensitivity of the photodiodes 8a. The sensitivity of the photodiodes 8a represents an amplitude difference of the electric signals 12a and 12b. FIG. 37 shows the differential output signal 11 in the case where amplitude of one electric signal 12a is smaller than that of the other electric signal 12b. Accordingly, it can be seen that the difference in sensitivity of the photodiodes 8a changes the duty cycle of the differential output signal 11 as shown in FIG. 37. However, the period T of the differential output signal 11 is not influenced and therefore equal to the period T of the differential output signal 11 shown in FIG. 36. Consequently, the differential output signal 11 of the encoder 6 has the most accurate period T (refer to Japanese Unexamined Patent Publication JP-A 2002-34274, for example).
Even though there is a difference in sensitivity of the photodiodes as described above, the output signal of the encoder has the most accurate period. Accordingly, in order to obtain more precise speed information, a simple-edge sampling method is adopted in which a period is counted from one rise to next rise of output signal, for example.
Further, in order to obtain more precise speed information and position information even when the photodiodes are different in sensitivity, a method is adopted of sampling both edges of each of two differential output signals which are out of phase with each other.
Taking a paper-feed control in the inkjet recording apparatus as an example, a sheet is fed at high speed at first, and a low-speed servo control is started at a short distance before a stop position. After that, the mode is shifted to a stop mode just before the target stop position so that the sheet is stopped at the target position. In this case, the sheet stopping accuracy is highly influenced by stabilization of a constant-speed servo control at the short distance before the stop position.
In driving the ASF motor at low speed as mentioned above, the output signal of the encoder will have smaller changes, thus resulting in longer update intervals of the position information and the speed information. As a result, the position information is not updated when the updating interval of the position information is longer than the servo period. In this case, it is determined that the sheet has stopped, and the speed information is controlled to be zero.
If a recording sheet has not reached a target position at this time, higher command voltage is outputted to the ASF motor to move the recording sheet to the target position. However, an actual speed may not be zero and in such a case, there is a problem of unstable servo control such that the speed may be so high for the recording sheet to be fed over the target position.
The unstable servo control impedes the recording sheet from being printed at desired position, and problems are caused, for example, that in the case where the recording sheet fails to stop at the target position and stops over the target position, a gap will appear between print data, while in the case where the recording sheet stops before the target position, the print data will overlap each other. Moreover, in the case where the recording sheet has reached far over the target position, the recording sheet will need to be moved in the opposite direction and back to the target position, resulting in lower print speed.
Further, in the case where the speed information is controlled so as not to be zero when the position information is not updated, the speed information will not be zero even when the actual speed is zero, therefore causing a problem of unstable servo control as described above.